In recent years, people have been paying more attention to maintenance and improvement of their health than before. People want to readily obtain information such as blood pressure, an electrocardiogram, a heart rate, and so on for health management. As such, Patent Document 1 discloses a portable blood pressure measuring device that measures an electrocardiographic wave and a plethysmogram to obtain a plethysmogram propagation time (pulse wave propagation time), and calculates a maximal blood pressure and a minimal blood pressure. This portable blood pressure measuring device includes an electrocardiographic wave detection means that has one electrode in contact with a biological body surface and detects an electrocardiographic potential (electrocardiographic wave) induced on the above electrode and a plethysmogram detection means that measures a pulse using a photoelectric sensor in contact with the biological body surface; obtains a pulse wave propagation time from a time difference between an R-wave (peak) of the detected electrocardiographic wave and a peak (rising point) of the detected pulse wave; and calculates blood pressures based on a correlation table of the pulse wave propagation times and the blood pressures defined in advance.
Because biological signals of electrocardiographic waves, photoelectric pulse waves, and the like are weak in magnitude and include a lot of noise depending on measurement environments, it is difficult to obtain precise measurement results. As such, a method in which filtering is performed using a frequency filter or the like to improve a signal to noise ratio (S/N ratio) is generally employed. Here, the above-mentioned electrocardiographic wave detection means configuring the portable blood pressure measuring device obtains an electrocardiographic wave signal via a notch filter that cuts AC noise of 50 through 60 Hz and a band pass filer that selectively passes frequency components of the electrocardiographic wave. The plethysmogram detection means extracts pulsation components (plethysmogram signal) via a band pass filter that selectively passes 0.3 to 10 Hz signals as the frequency components of the pulse wave.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-81285
There exists a problem that peaks of an electrocardiographic signal and a photoelectric pulse wave signal are shifted respectively when filtering is performed. To obtain a pulse wave propagation time with high precision, the peak of the electrocardiographic signal and the peak of the photoelectric pulse wave signal need be precisely detected. However, with the above-mentioned portable blood pressure measuring device, the peaks are shifted by the filtering being performed, which raises a risk that the precision in measurement of the pulse wave propagation time is lowered. In particular, filters generally have frequency characteristics (frequency dependency); meanwhile, electrocardiographic signals and photoelectric pulse wave signals have different frequency components from each other. As such, an amount of shift of the peak of the electrocardiographic signal and an amount of shift of the peak of the photoelectric pulse wave signal due to filtering are different from each other in some case. As a result, there arises a risk that the precision in measurement of the pulse wave propagation time is further lowered.